Click
Here
GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 22, GB4010, doi:10.1029/2007GB003136, 2008
for
Full
Article
The accumulation of silver in marine sediments: A link to biogenic Ba
and marine productivity
J. L. McKay1 and T. F. Pedersen2
Received 31 October 2007; revised 7 August 2008; accepted 15 August 2008; published 15 November 2008.
[1] The concentrations of Ag and a suite of redox-sensitive trace metals (Re, Cd, and
Mo) were measured in surface sediments from the Western Canadian, Mexican, Peruvian,
and Chilean continental margins. In all regions, Ag content increases from 80 ng g1
(i.e., lithogenic values) on the shelf up to as high as 1483 ng g1 on the lower slope.
However, the trend of increasing Ag with increasing water depth breaks down at
deepwater sites (>2500 m) where only lithogenic concentrations are documented. Silver
content does not correlate with the distributions of redox-sensitive trace metals,
suggesting that sedimentary redox conditions are not the primary control on Ag
accumulation. Instead, a positive correlation between Ag and Ba in surface and nearsurface sediments suggests that Ag is scavenged by and delivered to the sediment with the
organic particle flux. Scavenging probably results from the precipitation of Ag2S within
the organic particles due to the development of anoxia and sulfate reduction. If this
hypothesis is correct, then Ag has the potential to be a paleoproductivity proxy.
Citation: McKay, J. L., and T. F. Pedersen (2008), The accumulation of silver in marine sediments: A link to biogenic Ba and marine
productivity, Global Biogeochem. Cycles, 22, GB4010, doi:10.1029/2007GB003136.
1. Introduction
[2] It is becoming common practice to use the concentration or enrichment factor (i.e., concentration above lithogenic background) of certain redox-sensitive trace metals in
sediments as paleoredox proxies [e.g., Dean et al., 1999;
Zheng et al., 2000; Adelson et al., 2001; Ivanochko and
Pedersen, 2004; Nameroff et al., 2004; McKay et al., 2005;
Dean et al., 2006, Dean, 2007]. This application is often
based on the assumption that the primary mechanism of
metal accumulation in the sediment is the diffusion of
dissolved metal species from the overlying water column
into the sediment and their fixation under specific redox
conditions, either by precipitation or adsorption. Molybdate
(MoO2
4 ), for example, diffuses into sediments where, in the
presence of >11 mmol H2S, it is rapidly converted to
thiomolybdate (MoS2
4 ) and subsequently scavenged by
Fe sulfides [Bertine, 1972; Helz et al., 1996; Erickson
and Helz, 2000] leading to its enrichment. While important,
such diagenetic inputs are not the only vector by which
metals are added to sediments. Lithogenic, biogenic, and
scavenged fluxes also contribute. The lithogenic (or detrital)
component can be estimated by assuming that the metal
concentration or metal/Al ratio is similar to that of average
shale (or average crustal material) or by directly measuring
the concentration in detrital material collected proximal to
1
College of Oceanic and Atmospheric Sciences, Oregon State
University, Corvallis, Oregon, USA.
2
School of Earth and Ocean Sciences, University of Victoria, Victoria,
British Columbia, Canada.
Copyright 2008 by the American Geophysical Union.
0886-6236/08/2007GB003136$12.00
the study area. The biogenic and scavenged fluxes, collectively referred to as the nonlithogenic particle flux are
poorly characterized, but can be significant. For example,
in the eastern tropical North Pacific off Mexico at least 58%
of the Cd in sinking particles is nonlithogenic [Nameroff et
al., 2002] and Zheng et al. [2002] estimated that nonlithogenic U can constitute as much as 70% of the total U
in anoxic sediments.
[3] Silver readily forms an extremely insoluble sulfide
(pK 36) [Dyrssen and Kremling, 1990] and thus it is
assumed that natural (i.e., nonanthropogenic) enrichment
above typical lithogenic values of 80 ng g1 [Taylor and
McLennan, 1995] is restricted to anoxic sediments where
Ag2S precipitates. However, we will show here that not all
anoxic sediments are characterized by high Ag concentrations and that weakly suboxic sediments can have exceptionally high natural Ag concentrations. These observations
are based on the analysis of multicores and box cores
collected from four regions located along the eastern margin
of the Pacific Ocean (i.e., Western Canadian, Mexican,
Peruvian and Central Chilean margins; Figure 1). All four
regions are characterized by wind-driven coastal upwelling
and corresponding high rates of primary productivity (400 g
C m2 a1 on the western Canadian Margin [Antoine et al.,
1996]; 330 g C m2 a1 on the Mexican Margin [Longhurst
et al., 1995]; 350 g C m2 a1 on the Peruvian Margin
[Muller and Suess, 1979]; and 960 g C m2 a1 on the
Central Chilean Margin [Daneri et al., 2000]). However,
along the Western Canadian and Central Chilean margins
upwelling is seasonal (late spring to early fall) while on the
Peru and Mexican margins it occurs throughout the year.
The thickness and intensity of the oxygen minimum zone
(OMZ), defined here as that portion of the water column
GB4010
1 of 17
GB4010
MCKAY AND PEDERSEN: ACCUMULATION OF Ag IN MARINE SEDIMENTS
GB4010
Figure 1. Locations of coring sites within each of the four study regions. Note that two cores RR77mc
(Peru Margin) and RR31mc (Central Chilean Margin) are located south of the areas shown on their
respective maps.
with 0.5 ml l1 dissolved oxygen, also differs greatly from
region to region (Figure 2). Off western Canada the OMZ is
relatively deep (750 to 1300 m) and the lowest measured
oxygen concentration at the time of sample collection was
0.3 ml l1 at 920 m water depth (Site JT09). In contrast, the
oxygen minima off Mexico and Peru are relatively shallow
(80 to 800 m and 125 to 750 m, respectively) and in
both regions there is a zone within the OMZ that is characterized by denitrification (i.e., where O2 is <0.05 ml l1).
Off central Chile the OMZ is also shallow and quite thin
(125 to 350 m). At this location, oxygen concentration
varies seasonally from 0 to 5 mmol [Fossing et al., 1995]
and is typically <2 mmol (0.05 ml l1) during upwelling
periods [Ferdelman et al., 1997]. However, during El Nino
events upwelling of oxygen-rich water results in substantially higher oxygen concentrations on the shelf and upper
slope (e.g., 18 to 40 mmol or 0.4 to 1.0 ml l1 in March 1998)
[Schubert et al., 2000].
[4] Previous studies in these regions have focused on
understanding the geochemistry of redox-sensitive metals
such as Re, U, Cd, and Mo (Western Canadian Margin,
McKay et al. [2007]; Mexican Margin, Nameroff et al.
[2002]; Peruvian Margin, Morford [1999], Böning et al.
[2004], and McManus et al. [2006]; Chilean Margin,
Morford [1999], Böning et al. [2005], and McManus et al.
[2006]). With the exception of three recent papers [Böning
2 of 17
GB4010
MCKAY AND PEDERSEN: ACCUMULATION OF Ag IN MARINE SEDIMENTS
GB4010
Figure 2. Dissolved oxygen profiles for (a) slope sites and (b) shelf sites from each study region. Data
for the Western Canadian Margin were collected at sites JT02 (lower slope) and JT01 (shelf) during the
1996 Canadian Joint Global Ocean Flux Study (CJGOFS) research cruise. Data can be obtained from the
first author of this paper. Dissolved oxygen data for all other regions were obtained from the World Ocean
Database (National Oceanographic Data Center).
et al., 2004, 2005; Morford et al., 2008] there is little
information in the literature about the geochemistry of Ag
in continental margin sediments. The primary objective of
this study is to improve our understanding of the biogeochemical controls on Ag accumulation in marine sediments.
Implications that such controls have on the potential use of
Ag as a paleoproductivity proxy will also be discussed.
2. Sampling and Analytical Methods
2.1. Sample Collection
[5] In each study region multicores and/or box cores were
collected along transects extending down the slope through
the OMZ. Where possible cores were also obtained from the
shelf and deepwater sites (Table 1). Samples from the
Western Canadian Margin were collected in 1996 during a
Canadian JGOFS research cruise on the Canadian Coast
Guard ship John P. Tully. The Mexican Margin samples were
collected in 1990 during a cruise of the Scripps Institute R/V
New Horizon. Samples from the Peruvian and Central Chilean margins were collected in 1997 during an ODP site
survey cruise of the Scripps Institute R/V Roger Revelle.
Additional samples from the Central Chilean Margin were
obtained during the 1999 Thioploca-Chile expedition, conducted jointly by the Universidad de Concepción (Chile) and
Max Plank Institute for Marine Microbiology (Germany),
using the Universidad de Concepción research vessel Kay
Kay and the Chilean naval vessel V. Gormaz.
2.2. Analytical Methods
[6] Surface sediment samples (0 – 0.5 cm or 0 – 1.0 cm
depth interval) were taken from all of the cores. Downcore
samples were also collected from the Western Canadian
Margin cores using a similar sampling resolution. Samples
were then freeze dried and hand ground using an agate
pestle and mortar.
[7] Bulk sediments were microwave digested following
the method described by McKay et al. [2007]. The concentrations of Ag, Re, Cd, and Mo were then measured by
isotope dilution using inductively coupled plasma mass
spectrometry (VG PQ2+ quadrapole ICP-MS at the
University of British Columbia and Thermo X-Series II
quadrupole ICP-MS at the University of Victoria). The
relative standard deviation (r.s.d., 1 sigma) of the measurements, determined by repeated analysis of the National
Bureau Standard MESS-2 and the University of British
Columbia laboratory standard Saanich Bulk sediment
(SNB), was 10% (Ag), 13% (Re), 14% (Cd), and 9% (Mo).
With regards to accuracy, Ag and Cd analyses yielded values
(174 ± 17 ng g1 and 0.24 ± 0.03 mg g1, Ag and Cd
respectively) essentially identical to the certified values for
MESS-2 (e.g., 180 ± 20 ng g1 and 0.24 ± 0.01 mg g1, Ag
and Cd respectively), but Mo values (2.37 ± 0.21 mg g1)
were typically lower than the certified value of 2.85 ±
0.12 mg g1. There is no certified value for Re in MESS-2
with which to compare our average value of 2.88 ±
0.36 ng g1 (n = 33). However, our Re results, as well as
those for Cd and Mo, for the Mexican Margin samples are
essentially identical to measurements previously made by
Nameroff et al. [2002], suggesting that our analytical
approach for this suite of elements is sound.
[8] Major (Al) and minor (Zr, Mn, and Ba) element
contents were measured by one of three methods. When
sufficient material was available, concentrations were measured by X-ray fluorescence (XRF) following the procedure
3 of 17
MCKAY AND PEDERSEN: ACCUMULATION OF Ag IN MARINE SEDIMENTS
GB4010
GB4010
Table 1. Sampling Locations
Western Canadian
Margin (JT96)
JT01mc
JT04mc
JT06bc
JT09mc
JT02mc
JT05bc
Mexican Margin
(NH90)
NH19bc
NH3bc
NH1bc
NH2bc
NH11bc
NH7bc
NH12bc
NH15bc
NH6bc
NH17bc
Latitude
48°
49°
48°
48°
49°
49°
0
Longitude
N
N
N
N
N
N
45.95
00.710
58.730
54.760
12.810
07.910
Latitude
22°
22°
22°
22°
22°
22°
22°
22°
22°
22°
22.20
43.50
56.30
43.20
30.50
42.90
41.70
41.30
36.70
18.00
N
N
N
N
N
N
N
N
N
N
125°
126°
126°
126°
127°
127°
0
29.57
49.820
52.680
53.440
18.570
33.120
W
W
W
W
W
W
Longitude
106°
106°
106°
106°
106°
106°
106°
106°
106°
106°
15.20
17.40
26.20
21.60
17.50
27.50
27.80
29.00
31.10
33.10
W
W
W
W
W
W
W
W
W
W
Water Depth
(m)
Bottom Water Oxygena
(ml l1)
Sedimentation Ratesa
(cm ka1)
120
407
720
920
1340
1750
2.4
1.0
0.4
0.3
0.4 (1240 m)
1.2
39.5
0.7
1.6
4.7
6.3
4.6
Water Depth
(m)
Bottom Water Oxygenb
(mmol l1)
Sedimentation Ratesb
(cm ka1)
Surface Redox Condition
97
107
110
133
135
190
322
425
620
785
40
32
36
na
16
8
<5
<5
<5
<5
na
102
na
na
na
40
16
33
51
na
suboxic
suboxic
suboxic
suboxic
suboxic
anoxic
anoxic
anoxic
anoxic
anoxic
Surface Redox Condition
weakly
weakly
weakly
weakly
weakly
weakly
suboxic
suboxic
suboxic
suboxic
suboxic
suboxic
Peruvian Margin
(RR97)
Latitude
Longitude
Water Depth
(m)
Bottom Water Oxygenc
(mmol l1)
Sedimentation Rates
(cm ka1)
Surface Redox Condition
RR82mc
RR80mc
RR83mc
RR77mc
13° 420 S
13° 290 S
13° 100 S
16.134
76° 420 W
76° 530 W
77° 150 W
76.977
264
448
1419
2588
<10
8
na
139
na
na
na
na
anoxic
anoxic
suboxic
oxic
Bottom Water Oxygenc,d
(mmol l1)
Sedimentation Ratesc,d
(cm ka1)
Surface Redox Condition
<2
<2
<2
14
63
70
na
164
100 to 220
100 to 220
100 to 220
na
na
na
na
na
anoxic
anoxic
anoxic
anoxic
suboxic
suboxic
suboxic
oxic
Chilean Margin
(VG99 and RR97)
Latitude
Longitude
Water Depth
(m)
VG07mc
VG18mc
VG26mc
RR34mc
RR39mc
RR42mc
VG41mc
RR31mc
36° 370 S
36° 310 S
36° 260 S
36° 320 S
36° 10.30 S
36° 10.00 S
36° 200 S
37° 40.440
73° 010 W
73° 080 W
73° 230 W
73° 26.80 W
73° 34.330 W
73° 40.920 W
73° 490 W
75° 25.860 W
37
87
122
133
510
1028
2000
3923
a
McKay et al. [2007].
Nameroff et al. [2002].
RR97 data are from McManus et al. [2006] and J. McManus (unpublished data, 2008).
d
VG99 oxygen data are from Ferdelman et al. [1997] and sedimentation rate data are from Schubert et al. [2000].
b
c
described by Calvert et al. [1985]. Precision, determined
using various international rock standards (e.g., BEN,
BHVO-1, GSP-1, BIR1, JA-2, and JB-3), was 2% for
Al, 4% for Mn and Zr, and 6% for Ba, and the accuracy was
11% or better for all elements. When limited sample was
available, analysis involved fusing 200 mg of material with
900 mg LiBO2 at 1000°C and then dissolving the resulting
glass in 10% environmental grade HNO3. Major elements
were then measured by ICP-OES and minor elements by
ICP-MS using aliquots of the same solutions. The precision
(r.s.d., 1 sigma) of these measurements, assessed by running
the international rock standards JB-2 and SY-4, was 3% for
Al and 8% for Mn, Zr, and Ba.
[9] The concentration of lithogenic Ba in surface sediments from the Western Canadian Margin was directly
measured by XRF for six samples after the biogenic Ba had
been chemically extracted using 2M NH4Cl [Schenau et al.,
2001]. The biogenic Ba content was then calculated by
difference (biogenic Ba = total Ba – lithogenic Ba).
[10] Total carbon was measured using a Carlo-Erba
NA-1500 elemental analyzer and has a precision of 3% or
better and an accuracy of 5% or better based on the analysis of
PACS-1, MESS-1, and BCSS-1 with each batch of samples.
The percent carbonate carbon was determined by coulometry. A calcium carbonate standard that was analyzed
repeatedly with each batch of samples yielded a mean value
of 11.93 ± 0.13%. The percent organic carbon was calculated by difference (organic C = total C – carbonate C) and
has an associated error of 4% (r.s.d., 1 sigma). Biogenic
silica was measured using the Na2CO3 dissolution method
of Mortlock and Froelich [1989]. The error determined
using the opal-rich standard SNB is 4% (r.s.d., 1 sigma),
but is probably somewhat higher than this when opal
content is <10%.
3. Results
3.1. Sediment Description
[11] Surface sediments on the Western Canadian Margin
range from homogeneous, olive green muds (cores JT01,
09, 02, and 05) to sandy muds (JT06) and muddy sands
(JT04). Mexican margin sediments typically comprise olive
green silty clays and within the OMZ the deposits are finely
laminated. Surface sediments at sites RR82 and RR83 on
4 of 17
GB4010
MCKAY AND PEDERSEN: ACCUMULATION OF Ag IN MARINE SEDIMENTS
GB4010
Figure 3. Metal/Al ratios for Ag (solid circles), Re (squares), Cd (triangles), and Mo (diamonds) in
surface sediments collected from the (a) Western Canadian Margin excluding Site JT04 where metal
concentrations are below the detection limit, (b) Mexican Margin, (c) Peruvian Margin, and (d) Central
Chilean Margin. Data have been plotted as metal/Al ratios to remove the effects of variable terrigenous
input; however, the concentrations of Ag (in ng g1) are also provided within the brackets next to the
corresponding Ag/Al ratio. Typical lithogenic metal/Al ratios for bulk continental crust are Ag/Al = 9.5 107, Re/Al = 0.05 107, Cd/Al = 0.01 104, and Mo/Al = 0.12 104 [Taylor and McLennan, 1995].
the Peruvian Margin are homogeneous, black silty clays. At
Site RR80 the upper 2 cm is composed of dark gray sand
that is underlain by a laminated, dark gray mud. Laminations are also present below 10 cm in multicore RR82. At
the deepest site on the Peruvian Margin (RR77) the upper
4 cm consist of an oxidized, gray-brown silty clay. Surface
sediments from the Chilean shelf are olive brown, clayey to
silty muds and on the slope silty clays. No laminations were
observed in the Chilean cores.
3.2. Surface Sediment Data
[12] Geochemical data for surface sediments are provided
in Table S1 of the auxiliary material.1 The Ag and Ba data
1
Auxiliary materials are available in the HTML. doi:10.1029/
2007GB003136.
5 of 17
GB4010
MCKAY AND PEDERSEN: ACCUMULATION OF Ag IN MARINE SEDIMENTS
GB4010
Figure 4. (a) Mn/Al and (b) Ba/Al ratios for surface sediments. The average shale Mn/Al ratio is
indicated by the dashed line in Figure 4a.
for all four locations are new (i.e., not previously published). The organic carbon, opal, carbonate, as well as
major, minor and trace element data for the Peruvian and
Chilean margins are also new. Redox-sensitive trace metal
data and supporting organic carbon, opal, carbonate, and
major and minor elements concentration data for the Western
Canadian Margin were previously published by McKay et al.
[2007]. Similar supporting data for the Mexican Margin are
from Ganeshram [1996].
[13] The concentration of Ag in the surface sediments is
highly variable (<100 ng g1 up to 1483 ng g1). In
general, shelf sediments are characterized by the lowest
Ag concentrations and Ag/Al ratios similar to lithogenic
values (i.e., 9.5 107) [Taylor and McLennan, 1995]
while the highest Ag concentrations and Ag/Al ratios
generally occur on the lower slope and, more importantly,
below the OMZ (Figures 3a – 3d). This yields a trend of
increasing Ag with increasing water depth down the slope.
However, this relationship does not hold true at deepwater
sites (i.e., water depths >2500 m) that are characterized by
relatively low concentrations of Ag (<200 ng g1), similar
to the values previously reported for pelagic sediments
[Koide et al., 1986]. Furthermore, there is evidence of an
anomalous Ag enrichment (i.e., higher concentrations than
expected based on water depth) within the upper OMZ on
the Peruvian and Central Chilean margins (Figures 3c and
3d). Nevertheless, surface sediments from the lower slope at
both of these locations have Ag concentrations and Ag/Al
ratios as high as, or higher than, the OMZ sediments. The
general trend of increasing Ag with increasing water depth
was previously reported for Chilean Margin surface sediments [Böning et al., 2005] and sediments from the Northeast Pacific [Morford et al., 2008], but was not observed on
the Peruvian Margin [Böning et al., 2004].
[14] In contrast to Ag, the ratios of Re/Al, Cd/Al, and
Mo/Al (i.e., the "typical" redox-sensitive trace metals) do
not increase with water depth. These ratios, are either
invariant with water depth (e.g., Cd/Al and Mo/Al on the
Western Canadian Margin; Figure 3a) or exhibit maxima
within the OMZ (e.g., Mexican, Peruvian, and Central
Chilean margins; Figures 3b– 3d).
[15] At most sites Mn/Al ratios in surface sediments are
less than the average shale value of 0.0106 (Figure 4a)
indicating the dissolution of Mn oxyhydroxides under
reducing (i.e., suboxic) conditions. The exceptions are
deepwater sites RR77 and RR31 where Mn/Al ratios are
similar to the average shale value indicating oxic conditions
(Figure 4a). Interestingly, Ag concentrations and Ag/Al
ratios are relatively low (i.e., producing a reversal in the
trend of increasing Ag with increasing water depth) at these
same locations.
[16] Barium content and Ba/Al ratios in surface sediments
generally increase with water depth in all of the study
regions (Figure 4b). This relationship between Ba and water
depth was previously noted for sediments deposited on the
Mexican Margin [Nameroff et al., 2002], the Peruvian
Margin [von Breymann et al., 1992], and the Chilean
Margin [Klump et al., 2000]. Selective dissolution experi-
6 of 17
GB4010
MCKAY AND PEDERSEN: ACCUMULATION OF Ag IN MARINE SEDIMENTS
GB4010
Figure 5. Ag/Al ratios versus the concentrations of (a) Ba, (b) organic carbon, (c) calcium carbonate,
and (d) opal in surface sediments from the Western Canadian Margin (open circles), Mexican Margin
(solid squares), Peruvian Margin (crosses), and Central Chilean Margin (open triangles).
ments using surface sediments from the Western Canadian
Margin indicate that biogenic Ba concentrations increase
with water depth (e.g., from 0 mg g1 at 120 m up to
655 mg g1 at 1750 m) while the lithogenic Ba content of
surface sediments remains relatively constant. Given that
both Ba and Ag increase with water depth it is not
unexpected that there is a strong correlation between Ag/
Al ratios and Ba (r2 > 0.75 except on the Peruvian Margin)
for sediments deposited above 2500 m water depth
(Figure 5a). Below this depth, the correlation breaks down
because Ag concentrations decline to lithogenic levels while
Ba concentrations continue to increase. On the Peruvian
Margin the weak correlation between Ag/Al and Ba (r2 =
0.25) appears to reflect the anomalously high Ag content of
surface sediments from the upper OMZ.
[17] The organic carbon content of surface sediments is
highly variable from one study region to the next, but there
is one consistent trend, concentrations are highest within the
OMZ at all locations (3.4%, Western Canadian Margin;
8.9%, Mexican Margin; 15.1%, Peruvian Margin; and 5.1%,
Central Chilean Margin). There also appears to be a strong
correlation between Ag/Al ratios and organic carbon content
on the Mexican (r2 = 0.86) and Peruvian (r2 = 0.91) margins
(Figure 5b). Carbonate content is highly variable from one
study location to another (Figure 5c). Low concentrations
(<2.0%) characterize the Western Canadian and Central
7 of 17
GB4010
MCKAY AND PEDERSEN: ACCUMULATION OF Ag IN MARINE SEDIMENTS
GB4010
Figure 6. Downcore changes in the concentrations of Ag and redox-sensitive trace metals (Re and Cd)
in multicores (mc) and box cores (bc) collected from the Western Canadian Margin. The average
lithogenic background value for Ag is indicated by the vertical dashed line. The lithogenic concentrations
of Re and Cd (not shown) are 0.5 ng g1 and 0.2 mg g1, respectively.
Chilean margins while sediments on the Mexican and
Peruvian margins have concentrations ranging from 2.8 to
29.3%. Because of limited sample availability, opal concentrations were only measured for the Western Canadian
and Central Chilean margins (2.5 to 8.9% and 8.4 to 14.3%;
respectively; Figure 5d). In both regions, the highest opal
concentrations occur in shelf sediments (sites JT01 and
VG18). Unlike Ba and organic carbon, there is no relationship between carbonate and opal contents and that of Ag
(Figures 5c and 5d).
3.3. Downcore Results
[18] The downcore Ag data (this study) and corresponding
Re, Cd, and Mo results [McKay et al., 2007] for multicores
and box cores collected from the Western Canadian
Margin are provided in Figure 6 and Table S2 of the
auxiliary material. Shelf core JT01 is characterized by a
low (i.e., lithogenic) concentration of Ag throughout the
core (Figure 6a). In comparison, cores JT09, JT02, and JT05
are enriched in Ag and exhibit relatively large (100 to
200 ng g1) downcore variations in Ag content (Figures 6b–
6d). These variations in Ag do not correspond to changes in
the concentrations of the redox-sensitive trace metals, however. For example, in cores JT02 and JT05 Ag concentrations are higher in the upper part of the core while Re and
Cd are enriched in the underlying sediments (Figures 6c and
6d). There are also no similarities between the downcore
concentration profile of Ag and those of organic carbon,
opal, and carbonate contents in any of the cores (Figures 7a –
7d). In contrast, Ag and Ba concentration profiles are very
similar in cores JT02 and JT05 (Figures 8c and 8d),
8 of 17
MCKAY AND PEDERSEN: ACCUMULATION OF Ag IN MARINE SEDIMENTS
GB4010
GB4010
Figure 7. Downcore changes in the Ag, opal, organic carbon, and carbonate concentrations in
multicores (mc) and box cores (bc) collected from the Western Canadian Margin.
suggesting that the Ag-Ba relationship observed in surface
sediments can persist after burial. Interestingly, however, in
core JT09 the downcore concentration profiles of Ag and
Ba are not similar (Figure 8b), possibly due to Ba loss (i.e.,
dissolution of biogenic barite) from these strongly suboxic
sediments. There are no downcore similarities between the
Ag and Ba profiles in core JT01 simply because this core
contains only lithogenic concentrations of these elements.
4. Discussion
4.1. Sedimentary Redox Conditions
[19] Understanding how sedimentary redox conditions
influence the accumulation of various trace metals has been
the focus of a great deal of research. Detailed summaries
can be found in the works by Crusius et al. [1996], Morford
and Emerson [1999], Nameroff et al. [2002], Tribovillard
et al. [2006], and Calvert and Pedersen [2007].
[20] Redox conditions in surface sediments can generally
be inferred using a combination of Mn/Al ratios and
concentrations (or metal/Al ratios) of certain redox-sensitive
metals. Typically oxic sediments have Mn/Al ratios similar
to, or higher than, the average shale value of 0.0106
[Turekian and Wedepohl, 1961] and high Mo/Al ratios
(i.e., greater than the lithogenic value of 0.12 104)
[Taylor and McLennan, 1995] reflecting the presence of
oxyhydroxides. However, only lithogenic Re/Al and Cd/Al
ratios are observed (0.05 107 and 0.01 104)
[Taylor and McLennan, 1995]. Weakly suboxic sediments
have Mn/Al and Mo/Al ratios below the average shale (or
crustal) value as a result of the reduction and dissolution of
Fe and Mn oxyhydroxides and also low (i.e., lithogenic)
Re/Al and Cd/Al ratios. In contrast, strongly suboxic sediments exhibit Re and Cd enrichments above lithogenic
values. Finally, anoxic sediments have low Mn/Al ratios
and generally high Mo/Al, Cd/Al, and Re/Al ratios indicating that Mo, Cd, and Re are enriched above lithogenic
9 of 17
GB4010
MCKAY AND PEDERSEN: ACCUMULATION OF Ag IN MARINE SEDIMENTS
GB4010
Figure 8. Downcore changes in Ag and Ba concentrations in multicores (mc) and box cores (bc)
collected from the Western Canadian Margin.
background values. On the basis of this simplified classification system we have inferred the redox conditions in the
surface sediments at each of the sampling sites (see Table 1).
The details are discussed below.
[21] Shelf and slope sediments on the Western Canadian
Margin are deposited under oxic conditions (bottom water
oxygen 0.3 ml l1), but become suboxic within millimeters of the sediment-water interface (i.e., oxygen penetration depths of <1 cm) [McKay et al., 2007]. The Mn/Al
ratios of the surface sediments (upper 0.5 or 1.0 cm) are
correspondingly low (i.e., less than the average shale value;
Figure 4a), reflecting the reduction and dissolution of Mn
oxyhydroxides. However, the metal/Al ratios of the redoxsensitive trace metals are also low (e.g., Cd/Al and Mo/Al;
Figure 3a) because near-surface sediments are only weakly
suboxic (i.e., perched between Mn and I reduction) [McKay
et al., 2007]. Such weakly suboxic conditions persist
decimeters below the sediment-water interface due to relatively low sedimentation rates and/or deep bioturbation
[McKay et al., 2007]. Enrichment of Re and Cd above
typical lithogenic values (0.5 ng g1 and 0.2 mg g1,
respectively) is observed at depth in cores collected from
within and below the OMZ (i.e., cores JT09, 02, and 05;
Figures 6b– 6d). However, near-surface sediments at these
sites never become fully anoxic and thus exhibit no Mo
enrichment [McKay et al., 2007].
[22] On the Mexican Margin, surface sediments deposited
above the OMZ are characterized by high Re/Al ratios but
low Mn/Al, Cd/Al, and Mo/Al ratios (Figure 3b). These
results indicate that above the OMZ sediments become
strongly suboxic, but not anoxic, within millimeters of the
sediment-water interface [Nameroff et al., 2002]. All redoxsensitive trace metals, including Mo, are enriched within the
OMZ (Figure 3b) indicating that surface sediments are
anoxic [Nameroff et al., 2002]. These results are consistent
with the lack of bioturbation and resultant preservation of
fine laminations. No surface sediment samples from below
the OMZ were analyzed during this study. However,
Nameroff et al. [2002] measured the concentrations of
redox-sensitive trace metals in a core from 1020 m water
10 of 17
GB4010
MCKAY AND PEDERSEN: ACCUMULATION OF Ag IN MARINE SEDIMENTS
depth and found only elevated Re concentrations indicating
suboxic conditions just below the sediment-water interface.
[23] On the Peruvian Margin, surface sediments from all
but the deepest site have low Mn/Al ratios (<0.0061;
Figure 4a) indicating that reducing conditions develop
within 1 cm of the sediment-water interface. At the deepest
location (RR77; Figure 4a) the Mn/Al ratio is much higher
(0.0101) and similar to the average shale value suggesting
that surface sediments are oxic. The high concentrations of
Re, Cd, and Mo and the correspondingly high metal/Al
ratios in surface sediments from the OMZ, particularly the
upper OMZ (Figure 3c), are indicative of anoxic conditions.
At Site RR83, located below the OMZ only Re is significantly enriched suggesting that surface sediments are
strongly suboxic. The absence of redox-sensitive trace metal
enrichments at the deepest site (Figure 3c) suggests sediments are oxic which is consistent with the Mn data.
[24] Surface sediments at all but the deepest site on the
Central Chilean Margin are characterized by Mn/Al ratios
well below the average shale value indicating reducing
conditions (Figure 4a). In general, Re, Cd, and Mo are
enriched in surface sediments deposited within the OMZ
(i.e., metal/Al ratios above lithogenic values; Figure 3d)
suggesting that anoxic conditions develop soon after deposition. However, the degree of trace metal enrichment is
relatively low in comparison to anoxic surface sediments
from the Peruvian Margin, a contrast that is consistent with
the low levels of sulfide retention in the Chilean Margin
deposits despite high sulfate reduction rates [Ferdelman et
al., 1997]. Dissolved sulfide in these sediments is readily
oxidized, both by the bacterium Thioploca spp., which
forms mats on the sediment surface [Fossing et al., 1995;
Ferdelman et al., 1997] and via extensive bioturbation and
the influx of oxygen-rich waters during nonupwelling
periods and El Nino events [Ferdelman et al., 1997]. At
sites located below the OMZ and down to a depth of
2000 m only Re is enriched (Figure 3d) in surface sediments
suggesting that suboxic, not anoxic, conditions develop
shortly after deposition. At the deepest location (RR31;
Figure 4a) surface sediments are oxic, as indicated by the
high Mn/Al ratio. The relatively high Mo/Al ratio at this
deepwater site most certainly reflects adsorption of Mo onto
Mn oxyhydroxides [Bertine and Turekian, 1973]. However,
the high Re content is unexpected given that Re should only
be enriched in strongly suboxic sediments and cannot be
explained with the available data.
4.2. Barium Biogeochemistry
[25] Barium is known to be enriched in sediments that
underlie highly productive surface waters [Goldberg and
Arrhenius, 1958; Dehairs et al. 1980, 1992] where it occurs
as the mineral barite. The positive correlation between barite
and organic carbon in sediment traps and marine sediments
[Dymond et al., 1992; Francois et al., 1995; Dymond and
Collier, 1996] has led to the use of barium as a paleoproductivity proxy.
[26] A large proportion of the biogenic barite found in
sediments is formed in the upper water column where labile
organic matter rapidly decays [Chan et al., 1977; Dehairs et
al., 1980, 1990; Bishop, 1988]. However, sediment trap
GB4010
data [Dymond and Collier, 1996] and Ra isotope studies
(228Ra/226Ra ratios) [van Beek et al., 2007] suggest that
some barite continues to form deeper in the water column.
We attribute the increase in Ba (and Ba/Al ratios) with water
depth that is observed in all of our study regions to this
continual formation of barite as particles settle. This is
however a contentious matter. Others have suggested that
the Ba depth relationship reflects differences in barite
preservation related to sedimentation rate [Dymond et al.,
1992], the degree of barite saturation in the overlying
bottom water [Schenau et al., 2001], and sedimentary redox
conditions [von Breymann et al., 1992; Falkner et al., 1993;
McManus et al., 1994, 1998], as well as possible differences
in the efficiency of barite formation.
[27] By normalizing Ba to Al, which is assumed to be
predominantly detrital in origin, we essentially remove the
effect of variable terrigenous input. Furthermore, if the
increase in Ba content was the result of decreasing dilution
and/or sediment focusing we would expect other biogenic
components (e.g., organic carbon) to be similarly affected.
This is not the case; only Ba and Ag concentrations
increase with water depth. In comparison, organic carbon
tends to be highest within the OMZ and opal contents are
highest on the shelf.
[28] Sulfate reduction in anoxic sediments can lead to
undersaturation and dissolution of barite [von Breymann et
al., 1992; Falkner et al., 1993; McManus et al., 1994].
Suboxic sediments that are sufficiently reducing to accumulate diagenetic U may also experience barite dissolution
[McManus et al., 1998]. This mechanism cannot explain the
pattern of Ba accumulation in surface sediments from the
Western Canadian Margin because the sediments are only
weakly suboxic and show no depletion of pore water sulfate
[McKay et al., 2007]. On the Mexican Margin sedimentary
Ba concentration increases with water depth despite the fact
that surface sediments within the OMZ are anoxic, implying
that the intensity of sulfate reduction in these deposits is
insufficient to render the pore waters undersaturated with
respect to barite.
4.3. Silver Biogeochemistry
[29] Sandstones, normal shales and oxic marine sediments
have Ag concentrations on the order of 80 to 100 ng g1
[Smith and Carson, 1977; Koide et al., 1986], similar to
average bulk continental crust (80 ng g1) [Taylor and
McLennan, 1995]. In comparison, black shales and anoxic
marine sediments are commonly enriched in Ag, with
reported concentrations ranging from 130 to 1540 ng g1
[Smith and Carson, 1977; Koide et al., 1986]. It has been
assumed that such enrichments result from the precipitation
of diagenetic Ag2S in much the same way that Cd is thought
to be enriched in marine sediments [Rosenthal et al., 1995]
or possibly as a silver selenide [Crusius and Thomson, 2003].
[30] It is clear from the results of this study that Ag can be
extremely enriched in sediments that are not anoxic. For
example, suboxic sediments located below the OMZ on the
Central Chilean Margin are enriched in Ag relative anoxic
sediments found within the OMZ (Figure 3d). Weakly
suboxic surface sediments from the midslope to lower slope
on the Western Canadian Margin, which have low Re, Cd,
11 of 17
GB4010
MCKAY AND PEDERSEN: ACCUMULATION OF Ag IN MARINE SEDIMENTS
and Mo concentrations, also have elevated Ag concentrations (223 to 357 ng g1) and correspondingly high Ag/Al
ratios (Figure 3a). These observations suggest that sedimentary redox conditions are not the primary control on Ag
enrichment in continental slope sediments.
[31] Sediment texture could influence sedimentary Ag
concentrations, given that fine-grained sediments have a
slightly higher Ag content than coarser-grained deposits
[Smith and Carson, 1977]. For example, on the Western
Canadian Margin sandy sediments on the upper slope at Site
JT04 have lower Ag concentrations than the muds at shelf
Site JT01 (50 versus 84 ng g1, respectively). However,
changes in grain size cannot explain the extremely large
increases in Ag (as much as 15 times background) with
increasing water depth on the Mexican and Peruvian margins
where changes in grain size are relatively minor.
[32] Anthropogenic influences can similarly be ruled out.
Most industrial Ag is discharged to the coastal marine
environment from point sources (e.g., sewage outfalls)
and this can lead to extremely high concentrations of both
dissolved Ag (e.g., 307 pmol in San Diego Bay) [SanudoWilhelmy and Flegal, 1992] and sedimentary Ag (e.g.,
500 ng g1 in Puget Sound) [Bloom and Crecelius,
1987] and up to 1300 ng g1 in the Strait of Georgia
[Gordon, 1997]. However, such enrichments occur proximal to the source. The simple fact that Ag concentrations
increase offshore and down the continental slope is inconsistent with this type of anthropogenic supply. Industrial Ag,
in the form of dust, is also supplied to the surface ocean
[Ranville and Flegal, 2005], but the amount is relatively
small and relatively widespread, and thus unlikely to have
produced the Ag distribution we observe in the sediments
from the eastern margin of the North and South Pacific.
[33] The observed pattern of Ag accumulation in continental margin surface sediments cannot be explained by
variations in diagenetic, lithogenic or anthropogenic inputs,
it must be related to differences in the biogenic and/or
scavenged particulate flux to the sediment. We explore this
possibility below.
[34] Dissolved Ag exhibits a nutrient-type depth profile in
the ocean (i.e., depleted in surface waters and increasing
concentration with water depth) similar to that of dissolved
Cu [Martin et al., 1983] and Si [Flegal et al., 1995; RiveraDuarte et al., 1999; Ndung’u et al., 2001; Zhang et al.,
2001; Zhang et al., 2004; Ranville and Flegal, 2005]. The
concentration of dissolved Ag in deep waters also increases
from the northern Atlantic Ocean (2.8 to 4.0 pmol, Rivera)
[Rivera-Duarte et al., 1999] to the North Pacific Ocean
(40.9 – 55.0 pmol) [Zhang et al., 2004] and Bering Sea
(104.5 pmol) [Zhang et al., 2004], as is typical for nutrienttype elements. On the basis of the similarity between the
dissolved Ag and Si profiles [Flegal et al., 1995; Zhang et
al., 2001] and work in Saanich Inlet, British Columbia,
Canada, it has been suggested that Ag is incorporated into
diatom frustules and later released as these as these particles
dissolve [Kramer, 2006]. This hypothesis is supported by
culture experiments in the laboratory that show that Ag can
be actively incorporated by organisms, notably diatoms
[Reinfelder and Fisher, 1991; Fisher and Wente, 1993;
Lee and Fisher, 1994]. However, the relationship between
GB4010
Ag and Si is nonlinear (i.e., dissolved Ag increases more
slowly than Si with water depth) [Zhang et al., 2001, 2004;
Ranville and Flegal, 2005]. This may be the result of
relatively slower regeneration of Ag [Zhang et al., 2001]
or it may suggest that the distribution of dissolved Ag is
affected by processes other than simple biological uptake in
surface waters and regeneration at depth.
[35] If Ag is transported from the surface ocean directly to
the sediment within sinking diatom frustules, then the Ag
content of surface sediments should positively correlate
with the concentration of opal, assuming that any dissolution that has occurred in the surface sediments is congruent.
However, on the basis of the limited data available, no such
relationship is observed (Figure 5d). In fact, samples from
the Western Canadian and Central Chilean shelves that have
the highest opal content have lowest Ag concentrations.
Furthermore, opal concentration on the Peruvian Margin
tends to decrease with increasing water depth [Böning et al.,
2004], opposite to the trend exhibited by our Ag data. These
results imply that Ag enrichment is not directly related to
the opal flux to the sediment, although the delivery of some
Ag within diatom frustules cannot be ruled out. Likewise,
there does not appear to be a relationship between Ag and
carbonate contents (Figure 5c). There is however a positive
correlation (r2 > 0.85) between Ag/Al ratios and organic
carbon on the Mexican and Peruvian margins (Figure 5b)
and also a positive correlation (r2 > 0.75) between Ag/Al
ratios and Ba in three of the four locations, when deepwater
sites RR31 and RR77 are excluded (Figure 5a). These data
suggest that Ag, like Ba, is continuously scavenged from
the water column by settling organic particles. However, it
cannot be the result of incorporation of Ag in barite since
Ag and Ba have different valences and dissimilar ionic radii.
The precipitation of discrete Ag sulfate, which is highly
soluble in marine waters (Ksp = 1.4 105), is also very
unlikely. Moreover, these elements behave differently in
surface sediments from deepwater sites (i.e., Ag decreases
dramatically while Ba continues to increase) implying
incorporation into different phases.
[36] In oxic marine waters dissolved Ag occurs predominantly as very stable chloride complexes (AgCl
2 and
AgCl
3 ) [Savenko and Tagirov, 1996]. Complexation with
dissolved organic ligands, such as is observed in freshwater
and estuarine environments [Wen et al., 1997], is limited in
the marine environment [Cowan et al., 1985; Miller and
Bruland, 1995]. However, Ag, like other class B metals, has
a high affinity for reduced sulfur functional groups. Thermodynamic calculations suggest that in seawater at S2
concentrations >0.2 nmol kg1 Ag2S is oversaturated and
should precipitate [Cowan et al., 1985]. Indeed, the wholesale removal of dissolved Ag from anoxic, sulfide-rich
waters of Saanich Inlet, British Columbia, has been documented [Kramer, 2006]. Could a similar process (i.e., the
precipitation of a highly insoluble Ag sulfide phase) be
occurring within settling organic particles?
[37] Studies of man-made and natural organic-rich particles (e.g., marine snow and fecal pellets) show that the
degradation of organic matter can lead to the development
of oxygen-depleted microenvironment in such particles, and
under certain circumstances anaerobic conditions and sul-
12 of 17
GB4010
MCKAY AND PEDERSEN: ACCUMULATION OF Ag IN MARINE SEDIMENTS
fide formation [Alldredge and Cohen, 1987, Shanks and
Reeder, 1993; Ploug et al., 1997]. The development of
anoxic microenvironments is favored in larger particles
[Alldredge and Cohen, 1987; Ploug et al., 1997] and by
the presence of a physical structure (e.g., diatom frustule)
that slows diffusion of oxygen into the particle [Shanks and
Reeder, 1993]. More importantly, such microenvironments
can form in particles that are in contact with well-oxygenated waters [Shanks and Reeder, 1993], although their
formation would be favored in oxygen-poor environments
(e.g., within the OMZ). Clearly, bacteria are involved in the
degradation of the organic matter and thus the development
of anoxic conditions, but it is also possible that they play a
more active role in Ag accumulation. Luoma et al. [1995,
and references therein] noted that the sequestration of Ag on
inorganic particle in brackish and marine environments,
increased when bacterial coatings were present.
[38] We propose that Ag is passively scavenged from
seawater by the precipitation of Ag2S within anoxic microenvironments that develop in sinking organic particles. If
this is correct, it presents a paradox because the formation of
barite and Ag2S should be mutually exclusive. The reduction of sulfate that yields HS for reaction with Ag+ should
encourage barite dissolution [von Breymann et al., 1992;
Falkner et al., 1993; McManus et al., 1994, 1998]. We
hypothesize that within the interior of sinking organic
particles anoxic conditions develop and Ag2S precipitation
occurs. Surrounding this ‘‘anoxic core’’ the particle remains
oxic due to contact with the surrounding oxygenated seawater. In this outer zone barite formation occurs, aided by
the outward diffusion and oxidation of sulfide generated
within the particle. The resulting very local supersaturation
may explain why barite crystallization occurs despite the
fact that seawater is in general undersaturated with respect to
barite [Church and Wolgemuth, 1972; Monnin et al., 1999;
Monnin and Cividini, 2006]. The mechanism proposed for
the accumulation of Ag may also explain the enrichment of
other redox-sensitive metals in settling particulate organic
matter (e.g., U) [Zheng et al., 2002].
[39] Although there is as yet no direct evidence to support
the hypothesis of Ag scavenging by decaying organic
particles a number of observations lend strong support.
Martin et al. [1983] noted that the concentration of Ag in
suspended particulate matter collected off the coast of
Mexico increased dramatically at the top of the OMZ.
Our data also suggest a link between OMZ intensity and
the amount of Ag that accumulates in sediments. Simply
stated, the stronger the OMZ the more Ag that accumulates
within and, more importantly, below the OMZ. Hence,
sediments from the Mexican and Peruvian margins, locations characterized by an exceptionally strong OMZ and
denitrification within the water column, have the highest Ag
enrichments. Scavenging of Ag from the water column
might also explain (1) why dissolved Ag is depleted relative
to Cd in the oxygen-poor intermediate waters of the
Northeast Pacific [Kramer, 2006], (2) why the highest
dissolved Ag concentrations in the Pacific occur below,
not within, the OMZ [Zhang et al., 2004], and (3) why there
is a nonlinear relationship between dissolved Ag and Si in
the North Pacific Ocean [Zhang et al., 2001, 2004]. Inter-
GB4010
estingly, suspended particulate barite concentrations in the
Southern Ocean are also inversely correlated to dissolved
oxygen in the water column [Dehairs et al., 1990, 1992,
1997] supporting the idea that lower oxygen favors both Ag
and Ba scavenging.
[40] Leading further credence to the hypothesis is the
observation that bottom water oxygen concentration
appears to influence the preservation of Ag following
deposition. Surface sediments from deepwater cores
(>2500 m water depth) are characterized by low (i.e.,
lithogenic) Ag concentrations. These same sediments contain low amounts of Re and Cd and have relatively high
Mn/Al ratios indicative of oxic conditions and consistent
with relatively high bottom water oxygen concentrations.
Under such conditions a reduced but labile phase such as
Ag2S should be readily oxidized and the Ag released into
the pore water. This mechanism could explain the extremely
high pore water concentrations of Ag in oxic, deepwater
sediments on the Washington/Oregon continental margin
[Morford et al., 2008].
4.4. Authigenic Ag Enrichment Within Sediments
[41] If Ag is enriched in sediments in a similar manner as
Ba (i.e., scavenging by settling organic particles), then it
might be possible to use sedimentary Ag concentrations as a
paleoproductivity proxy. However, if Ag2S can precipitate
within anoxic microenvironments that develop in settling
organic particles then it should just as easily precipitate in
reduced sediments, masking any productivity signal. There
is evidence of anomalous Ag accumulation (i.e., higher than
expected Ag enrichment given the water depth) within the
upper OMZ on both the Peruvian and Central Chilean
margins. This ‘‘extra’’ Ag may be diagenetic in origin given
that it corresponds to high Re, Cd, and Mo accumulations.
In general, however, shallow water sediments contain only
lithogenic Ag concentrations regardless of whether they are
weakly suboxic, strongly suboxic or anoxic. There is also
no evidence of Ag accumulation when strongly suboxic
conditions develop below the sediment-water interface in
cores from the Western Canadian Margin (e.g., below
16 cm in multicore JT09; Figure 6b) and anoxic Chilean
margin sediments (i.e., core A, 126 m water depth) [Böning
et al., 2005].
[42] To explain the apparent lack of diagenetic Ag it is
first necessary to calculate, using equation (1), how much
diagenetic Ag (Agdiag) should be expected.
Agdiag ¼ ðAgsw =Cdsw Þ Cddiag
ð1Þ
Where Agsw/Cdsw is the Ag/Cd ratio in seawater and Cddiag
is the diagenetic component (Cddiag = Cmeas Cdlith Cdbio, were Cdmeas, Cdlith, and Cdbio are the measured,
lithogenic, and biogenic Cd concentrations, respectively). It
is assumed that the geochemical behavior of Ag within the
sediment is similar to that of Cd (i.e., both precipitate as
sulfides when trace amounts of HS are available). In fact,
on the basis of their pK values (36 for Ag2S, Dyrssen and
Kremling [1990]; 14 for CdS, Daskalakis and Helz [1992])
Ag has a higher affinity for sulfide and thus diagenetic Ag
may be overestimated using equation (1). Other assumptions
13 of 17
MCKAY AND PEDERSEN: ACCUMULATION OF Ag IN MARINE SEDIMENTS
GB4010
GB4010
Table 2. Estimated Diagenetic Ag Concentrationsa
Diagenetic Ag (ng g1)
Water Depth
(m)
Ag/Cdb
10
20
30
40
50
75
100
150
200
300
400
600
800
1001
1500
2000
2500
3000
0.03
0.04
0.03
0.04
0.03
0.04
0.01
0.01
0.01
0.01
0.01
0.02
0.02
0.03
0.04
0.06
0.06
0.06
Weakly Suboxic
(0.1 mg g1 Cddiag)c
Strongly Suboxic
(1.0 mg g1 Cddiag)c
Weakly Anoxic
(3 mg g1 Cddiag)c
Anoxic
(10 mg g1 Cddiag)c
Strongly Anoxic
(40 mg g1 Cddiag)c
3
30
90
301
1205
4
38
114
379
1518
3
34
103
342
1367
1
407
1626
4
41
122 (VG07, 96 ng g )
3
32
96
319
1278
4
38
114
379
1515
128
511
1 (JT01, 84 ng g1) 13 (NH19, 197 ng g1) 38 (VG26, 104 ng g1)
1
11
32
107
428
1
11
32
107
428
1
12
36
120 (NH12, 581 ng g1) 479 (RR82, 760 ng g1)
500
1
12
37
125 (RR80, 320 ng g1)
2
16
48
161
645
2
23
69
230
919
26
79
263
1054
3 (JT09, 223 ng g1)
4
40 (RR83, 1483 ng g1)
6
6
6
-
a
Actual measured Ag concentrations for specific sample sites are given in brackets. These values have not been corrected for the presence of lithogenic
Ag.
b
Water column Ag/Cd ratios were measured on samples collected from the NE Pacific off Vancouver Island, Canada [Kramer, 2006].
c
Concentration of diagenetic Cd used in equation (1) to calculate diagenetic Ag.
are (1) that diagenetic Ag and Cd precipitate in roughly the
same proportion as they occur in the water column, (2) that
the amount of lithogenic Cd can be estimated and corrected
for, and (3) that the concentration of biogenic Cd is negligible
or can be corrected for.
[43] Equation (1) is applied to five hypothetical situations
representative of weakly suboxic, strongly suboxic and
anoxic (low, medium, and high Cd) surface sediments
(Table 2). It is obvious that very little diagenetic Ag precipitates in weakly suboxic surface sediments such as those from
the Western Canadian Margin (e.g., sites JT01 and JT09;
Table 2). The amount of diagenetic Ag that precipitates in
strongly suboxic surface sediments is detectable but low
(<41 ng g1; Table 2). Thus, most of the Ag enrichment
observed in the surface sediments at sites NH19 and RR83
is the result of water column scavenging. However, in
anoxic sediments, particularly those with a substantial Cd
enrichment, the concentration of diagenetic Ag is high (e.g.,
sites NH12, RR80, and RR82; Table 2).
[44] While downcore variations in Ag content do not
correlate with redox-sensitive trace metal concentrations,
they do covary with Ba in midslope to lower slope cores
from the Western Canadian Margin (e.g., cores JT02mc and
JT05bc; Figures 8c and 8d). This supports the hypothesis
that Ag and Ba are closely linked and suggests that Ag may
have some use as a paleoproductivity proxy. The fact that
Ag and Ba do not covary in core JT09mc, collected from
within the OMZ, is probably the result of poor Ba preservation due to the development of strongly suboxic conditions below 16 cm. This observation highlights a
potential advantage in using Ag, rather than Ba, to infer
changes in paleoproductivity in sediments that become
strongly reducing with depth, a feature characteristic of
many continental margin settings. However, Ag cannot be
used in sediments that are characterized by persistent oxic
conditions (i.e., in cores characterized by Mn/Al ratios
similar to that of average shale) due to oxidation and loss
of the reduced Ag phase. Nor is Ag useful in shallow water
sediments because there is insufficient time for settling
organic particles to scavenge Ag before they reach the
seafloor. Thus, shallow water cores should only contain
lithogenic concentrations of Ag (e.g., shelf core JT01mc;
Figure 8a) unless the sediments are strongly anoxic in
which case diagenetic Ag may be present.
5. Summary
[45] The Ag concentrations reported in this study are
among the highest ever reported for uncontaminated marine
sediments. More importantly, some of these extremely high
values (up to 15 times above lithogenic background values)
occur in suboxic (but not anoxic) marine sediments. This
observation and the fact that Ag enrichments do not
positively correlate with enrichments of redox-sensitive
trace metals (i.e., Re, Cd, and Mo) suggest that sedimentary
redox conditions are not the primary control on Ag accumulation. Variations in the lithogenic and anthropogenic
fluxes also are not controlling Ag. Thus, differences in
sedimentary Ag must be related to changes in the biogenic
and/or scavenged flux to the sediment. The very strong
positive correlation between Ag and Ba suggests that Ag,
like Ba, may be enriched in decaying organic particles as
they settle through the water column. We hypothesize that
within these particles anoxic microenvironments develop
as a result of organic degradation. This leads to the
formation of dissolved sulfide and the rapid precipitation
of Ag, most probably as Ag2S. The trend of increasing
sedimentary Ag concentrations going downslope most
probably reflects the longer residence time of organic
particles in the water column (i.e., more time for Ag
scavenging), possibly enhanced by higher concentrations
of dissolved Ag in deeper waters. How effectively Ag is
14 of 17
GB4010
MCKAY AND PEDERSEN: ACCUMULATION OF Ag IN MARINE SEDIMENTS
scavenged by particles appears to be related to oxygen
concentrations in the water column. Scavenging is more
effective, resulting in higher sedimentary Ag concentrations, in an environment characterized by an intense OMZ
(e.g., Mexican and Peru margins). This simply reflects the
fact that settling organic particles become anoxic more
rapidly when oxygen is limited, allowing more time for
Ag scavenging.
[46] This hypothesis will require testing, but if correct it
might be possible to use Ag as a paleoproductivity proxy, in
much the same way that Ba is used. There are caveats,
however. First, the host phase for silver, possibly Ag2S,
does not persist in oxygenated, deep sea sediments. Second,
the addition of Ag to anoxic sediments via the authigenic
precipitation of Ag2S can be substantial. We suggest that
with reasonable assumptions, this diagenetic component can
be estimated and corrected for using the sedimentary Cd
concentration. Finally, it appears that the amount of Ag
scavenged may be affected by the intensity of the OMZ
(i.e., enhanced scavenging as the OMZ becomes more
intense because settling particles go anoxic faster and
remain that way for longer). While this complicates the
use of Ag as a paleoproductivity proxy, it could be
employed to identify past changes in OMZ intensity. This
possibility and the potential use of Ag as a paleoproductivity
proxy will be discussed in a future paper.
[47] Acknowledgments. The authors gratefully acknowledge V.A.
Gallardo (University of Concepción), R. Ganeshram (Edinburgh University),
J. McManus, A. Mix (Oregon State University), and T. Nameroff for
providing samples and/or data for this study. The manuscript greatly
benefited from the reviews of two anonymous reviewers and the Editors E.
Saltzman and M. Andreae. Funding for this project was provided by the
Canadian Foundation for Climate and Atmospheric Science (CFCAS) and
the Natural Sciences and Engineering Research Council of Canada (NSERC)
through grants to T.F.P. and an NSERC graduate scholarship to J.L.M.
References
Adelson, J. M., G. R. Helz, and C. V. Miller (2001), Reconstructing the rise
of recent coastal anoxia: Molybdenum in Chesapeake Bay sediments,
Geochim. Cosmochim. Acta, 65, 237 – 252, doi:10.1016/S0016-7037(00)
00539-1.
Alldredge, A. L., and Y. Cohen (1987), Can microscale chemical patches
persist in the Sea? Microelectrode study of marine snow, fecal pellets,
Science, 235, 689 – 691, doi:10.1126/science.235.4789.689.
Antoine, D., J. M. Andre, and A. Morel (1996), Oceanic primary production: 2. Estimation at global scale form satellite (coastal zone colour
scanner) chlorophyll, Global Biogeochem. Cycles, 10, 57 – 69,
doi:10.1029/95GB02832.
Bertine, K. K. (1972), The deposition of molybdenum in anoxic waters,
Mar. Chem., 1, 43 – 53, doi:10.1016/0304-4203(72)90005-9.
Bertine, K. K., and K. K. Turekian (1973), Molybdenum in marine sediments, Geochim. Cosmochim. Acta, 37, 1415 – 1434, doi:10.1016/00167037(73)90080-X.
Bishop, J. K. B. (1988), The barite-opal-organic carbon association in
oceanic particulate matter, Nature, 332, 341 – 343, doi:10.1038/332341a0.
Bloom, N. S., and E. A. Crecelius (1987), Distribution of silver, mercury,
lead, copper and cadmium in central Puget Sound sediments, Mar. Chem.,
21, 377 – 390, doi:10.1016/0304-4203(87)90057-0.
Böning, P., H.-J. Brumsack, M. E. Böttcher, B. Schnetger, C. Kriete,
J. Kallmeyer, and S. L. Borchers (2004), Geochemistry of Peruvian nearsurface sediments, Geochim. Cosmochim. Acta, 68, 4429 – 4451,
doi:10.1016/j.gca.2004.04.027.
Böning, P., S. Cuypers, M. Grunwald, B. Schnetger, and H.-J. Brumsack
(2005), Geochemical characteristics of Chilean upwelling sediments at
36°S, Mar. Geol., 220, 1 – 21, doi:10.1016/j.margeo.2005.07.005.
Calvert, S. E., and T. F. Pedersen (2007), Elemental proxies for palaeoclimatic and palaeoceanographic variability in marine sediments: Interpretations and applications, in Proxies in Late Cenozoic Paleoceanography,
GB4010
edited by C. Hillaire-Marcel and A. de Vernal, pp. 567 – 644, Elsevier,
New York.
Calvert, S. E., B. L. Cousens, and M. Y. S. Soon (1985), An X-ray fluorescence spectrometric method for the determination of major and minor
elements in ferromanganese nodules, Chem. Geol., 51, 9 – 18, doi:10.1016/
0009-2541(85)90083-X.
Chan, L. H., D. Drummond, J. M. Edmond, and B. Grant (1977), On the
barium data from the Atlantic GEOSECS expedition, Deep Sea Res., 24,
613 – 649, doi:10.1016/0146-6291(77)90505-7.
Church, T. M., and K. Wolgemuth (1972), Marine barite saturation, Earth
Planet. Sci. Lett., 15, 35 – 44, doi:10.1016/0012-821X(72)90026-X.
Cowan, C. E., E. A. Jenne, and E. A. Crecelius (1985), Silver speciation in
seawater: The importance of sulfide and organic complexation, in Marine
and Estuarine Geochemistry, edited by A. C. Sigleo and A. Hatori, pp.
285 – 303, Lewis, Chelsea, Mich.
Crusius, J., and J. Thomson (2003), Mobility of authigenic rhenium, silver,
and selenium during postdepositional oxidation in marine sediments,
Geochim. Cosmochim. Acta, 67, 265 – 273, doi:10.1016/S0016-7037(02)
01075-X.
Crusius, J., S. Calvert, T. F. Pedersen, and D. Sage (1996), Rhenium and
molybdenum enrichment in sediments as indicators of oxic, suboxic and
sulfidic conditions of deposition, Earth Planet. Sci. Lett., 145, 65 – 78,
doi:10.1016/S0012-821X(96)00204-X.
Daneri, G., V. Dellarossa, R. Quinones, B. Jacob, P. Montero, and O. Ulloa
(2000), Primary production and community respiration in the Humboldt
Current System off Chile and associated oceanic areas, Mar. Ecol. Prog.
Ser., 197, 41 – 49, doi:10.3354/meps197041.
Daskalakis, K. D., and G. R. Helz (1992), Solubility of CdS (Greenockite)
in sulfidic waters at 25°C, Environ. Sci. Technol., 26, 2462 – 2468,
doi:10.1021/es00036a019.
Dean, W. E. (2007), Sediment geochemical records of productivity and
oxygen depletion along the margin of western North America during
the past 60,000 years: Teleconnections with Greenland ice and the Cariaco Basin, Quat. Sci. Rev., 26, 98 – 114, doi:10.1016/j.quascirev.
2006.08.006.
Dean, W. E., D. Z. Piper, and L. C. Peterson (1999), Molybdenum accumulation in Cariaco Basin sediment over the past 24 k.y.: A record of
water-column anoxia and climate, Geology, 27, 507 – 510, doi:10.1130/
0091-7613(1999)027<0507:MAICBS>2.3.CO;2.
Dean, W. E., Y. Zheng, J. D. Ortiz, and A. van Geen (2006), Sediment Cd
and Mo accumulation in the oxygen-minimum zone off western Baja
California linked to global climate over the past 52 kyr, Paleoceanography, 21, PA4209, doi:10.1029/2005PA001239.
Dehairs, F., R. Chesselet, and J. Jedwab (1980), Discrete suspended particles of barite and the barium cycle in the open ocean, Earth Planet. Sci.
Lett., 49, 528 – 550, doi:10.1016/0012-821X(80)90094-1.
Dehairs, F., L. Goeyens, N. Stroobants, P. Bernard, C. Goyet, A. Poisson,
and R. Chesselet (1990), On suspended barite and the oxygen minimum
in the Southern Ocean, Global Biogeochem. Cycles, 4, 85 – 102,
doi:10.1029/GB004i001p00085.
Dehairs, F., W. Baeyens, and L. Goeyens (1992), Accumulation of suspended barite at mesopelagic depths and export production in the Southern Ocean, Science, 258, 1332 – 1335, doi:10.1126/science.258.5086.1332.
Dehairs, F., D. Shopova, S. Ober, C. Veth, and L. Goeyens (1997), Particulate barium stocks and oxygen consumption in the Southern Ocean
mesopelagic water column during spring and early summer: Relationship
with export production, Deep Sea Res., Part II, 44, 497 – 516,
doi:10.1016/S0967-0645(96)00072-0.
Dymond, J., and R. Collier (1996), Particulate barium fluxes and their
relationships to biological productivity, Deep Sea Res., Part II, 43,
1283 – 1308, doi:10.1016/0967-0645(96)00011-2.
Dymond, J., E. Suess, and M. Lyle (1992), Barium in deep-sea sediments:
A geochemical proxy for paleoproductivity, Paleoceanography, 7, 163 –
181, doi:10.1029/92PA00181.
Dyrssen, D., and K. Kremling (1990), Increasing hydrogen sulphide concentrations and trace metal behaviour in the anoxic Baltic waters, Mar.
Chem., 30, 193 – 204, doi:10.1016/0304-4203(90)90070-S.
Erickson, B. E., and G. R. Helz (2000), Molybdenum (VI) speciation in
sulfidic waters: Stability and lability of thiomolybdates, Geochim. Cosmochim. Acta, 64, 1149 – 1158, doi:10.1016/S0016-7037(99)00423-8.
Falkner, K. K., G. P. Klinkhammer, T. S. Bowers, J. F. Todd, B. L. Lewis,
W. M. Landing, and J. M. Edmond (1993), The behavior of barium in
anoxic marine waters, Geochim. Cosmochim. Acta, 57, 537 – 554,
doi:10.1016/0016-7037(93)90366-5.
Ferdelman, T. G., C. Lee, S. Pantoja, J. Harder, B. M. Bebout, and
H. Fossing (1997), Sulfate reduction and methanogenesis in a Thioplocadominated sediment off the coast of Chile, Geochim. Cosmochim. Acta,
61, 3065 – 3079, doi:10.1016/S0016-7037(97)00158-0.
15 of 17
GB4010
MCKAY AND PEDERSEN: ACCUMULATION OF Ag IN MARINE SEDIMENTS
Fisher, N. S., and M. Wente (1993), The release of trace elements by dying
marine phytoplankton, Deep Sea Res., Part 1, 40, 671 – 694, doi:10.1016/
0967-0637(93)90065-B.
Flegal, A. R., S. A. Sanudo-Wilhelmy, and G. M. Scelfo (1995), Silver in
the eastern Atlantic Ocean, Mar. Chem., 49, 315 – 320, doi:10.1016/03044203(95)00021-I.
Fossing, H., et al. (1995), Concentration and transport of nitrate by the matforming sulphur bacterium Thioploca, Nature, 374, 713 – 715,
doi:10.1038/374713a0.
Francois, R., S. J. Honjo, S. J. Manganini, and G. Ravizza (1995), Biogenic
barium fluxes to the deep sea: Implications for paleoproductivity reconstruction, Global Biogeochem. Cycles, 9, 289 – 303, doi:10.1029/
95GB00021.
Ganeshram, R. (1996), On the glacial-interglacial variability of upwelling,
carbon burial and denitrification on the northwestern Mexican continental
margin, Ph.D. thesis, 233 pp., Univ. of British Columbia, Vancouver,
Canada.
Goldberg, E. D., and G. O. S. Arrhenius (1958), Chemistry of pelagic
Pacific sediments, Geochim. Cosmochim. Acta, 13, 153 – 198,
doi:10.1016/0016-7037(58)90046-2.
Gordon, K. (1997), Sedimentary tracers of sewage inputs to the southern
Strait of Georgia, M. S. thesis, Univ. of British Columbia, Vancouver,
Canada.
Helz, G. R., C. V. Miller, J. M. Charnock, J. F. W. Mosselmans, R. A. D.
Pattrick, C. D. Garner, and D. J. Vaughan (1996), Mechanism of molybdenum removal from the sea and its concentration in black shales: EXAFS
evidence, Geochim. Cosmochim. Acta, 60, 3631 – 3642, doi:10.1016/
0016-7037(96)00195-0.
Ivanochko, T. S., and T. F. Pedersen (2004), Determining the influences of
Late Quaternary ventilation and productivity variations on Santa Barbara
Basin sedimentary oxygenation: A multi-proxy approach, Quat. Sci. Rev.,
23, 467 – 480, doi:10.1016/j.quascirev.2003.06.006.
Klump, J., D. Hebbeln, and G. Wefer (2000), The impact of sediment
provenance on barium-based productivity estimates, Mar. Geol., 169,
259 – 271, doi:10.1016/S0025-3227(00)00092-X.
Koide, M., V. F. Hodge, J. S. Yang, M. Stallard, E. G. Goldberg, J. Calhoun,
and K. K. Bertine (1986), Some comparative marine chemistries of
rhenium, gold, silver and molybdenum, Appl. Geochem., 1, 705 – 714,
doi:10.1016/0883-2927(86)90092-2.
Kramer, D. (2006), An exploration of the marine silver cycle in coastal and
open ocean environments of the North Pacific, M. S. thesis, 115 pp.,
Univ. of British Columbia, Vancouver, Canada.
Lee, B., and N. Fisher (1994), Effects of sinking and zooplankton grazing
on the release of elements from planktonic debris, Mar. Ecol. Prog. Ser.,
110, 271 – 281, doi:10.3354/meps110271.
Longhurst, A. R., S. Sathyendranath, T. Platt, and C. Caverhill (1995), An
estimate of global primary production in the ocean from satellite radiometer data, J. Plankton Res., 17, 1245 – 1271, doi:10.1093/plankt/
17.6.1245.
Luoma, S. N., Y. B. Ho, and G. W. Bryan (1995), Fate, bioavailability and
toxicity of silver in estuarine environments, Mar. Pollut. Bull., 31, 44 –
54, doi:10.1016/0025-326X(95)00081-W.
Martin, J. H., G. A. Knauer, and R. M. Gordon (1983), Silver distributions
and fluxes in north-east Pacific waters, Nature, 305, 306 – 309,
doi:10.1038/305306a0.
McKay, J. L., T. F. Pedersen, and J. Southon (2005), Intensification of the
oxygen minimum zone in the Northeast Pacific off Vancouver Island
during the last deglaciation: Ventilation and/or export production?,
Paleoceanography, 20, PA4002, doi:10.1029/2003PA000979.
McKay, J. L., T. F. Pedersen, and A. Mucci (2007), Sedimentary redox
conditions in continental margin sediments (N.E. Pacific): Influence on
the accumulation of redox-sensitive trace metals, Chem. Geol., 238,
180 – 196, doi:10.1016/j.chemgeo.2006.11.008.
McManus, J., W. M. Berelson, G. P. Klinkhammer, T. E. Kilgore, and D. E.
Hammond (1994), Remobilization of barium in continental margin sediment, Geochim. Cosmochim. Acta, 58, 4899 – 4907, doi:10.1016/00167037(94)90220-8.
McManus, J., et al. (1998), Geochemistry of barium in marine sediments:
Implications for its use as a paleoproxy, Geochim. Cosmochim. Acta, 62,
3453 – 3473, doi:10.1016/S0016-7037(98)00248-8.
McManus, J., W. M. Berlson, S. Severmann, R. L. Poulson, D. E. Hammond,
G. P. Klinkhammer, and C. Holm (2006), Molybdenum and uranium
geochemistry in continental margin sediments: Paleoproxy potential,
Geochim. Cosmochim. Acta, 70, 4643 – 4662, doi:10.1016/j.gca.2006.
06.1564.
Miller, L. A., and K. W. Bruland (1995), Organic speciation of silver in
marine waters, Environ. Sci. Technol., 29, 2616 – 2621, doi:10.1021/
es00010a024.
GB4010
Monnin, C., and D. Cividini (2006), Saturation state of the world’s ocean
with respect to (Ba, Sr) SO4 solid solutions, Geochim. Cosmochim. Acta,
70, 3290 – 3298, doi:10.1016/j.gca.2006.04.002.
Monnin, C., C. Jeandel, T. Cattaldo, and F. Dehairs (1999), The marine
barite saturation state of the world’s oceans, Mar. Chem., 65, 253 – 261,
doi:10.1016/S0304-4203(99)00016-X.
Morford, J. L. (1999), The geochemistry of redox-sensitive trace metals,
Ph.D. thesis, 242 pp., Univ. of Washington, Seattle.
Morford, J. L., and S. R. Emerson (1999), The geochemistry of redox
sensitive trace metals in sediments, Geochim. Cosmochim. Acta, 63,
1735 – 1750, doi:10.1016/S0016-7037(99)00126-X.
Morford, J. L., L. H. Kalnejais, P. Helman, G. Yen, and M. Reinard (2008),
Geochemical cycling of silver in marine sediments along an offshore
transect, Mar. Chem., 110, 77 – 88, doi:10.1016/j.marchem.2008.02.008.
Mortlock, R. A., and P. N. Froelich (1989), A simple method for the rapid
determination of biogenic opal in pelagic marine sediments, Deep Sea
Res., 36, 1415 – 1426, doi:10.1016/0198-0149(89)90092-7.
Muller, P. J., and E. Suess (1979), Productivity, sedimentation-rate, and
sedimentary organic-matter in the oceans 1: Organic-carbon preservation, Deep Sea Res., Part A, 26, 1347 – 1362, doi:10.1016/01980149(79)90003-7.
Nameroff, T. J., L. S. Balistrieri, and J. W. Murray (2002), Suboxic trace
metal geochemistry in the eastern tropical North Pacific, Geochim.
Cosmochim. Acta, 66, 1139 – 1158, doi:10.1016/S0016-7037(01)00843-2.
Nameroff, T. J., S. E. Calvert, and J. W. Murray (2004), Glacial-interglacial
variability in the eastern tropical North Pacific oxygen minimum zone
recorded by redox-sensitive trace metals, Paleoceanography, 19, PA1010,
doi:10.1029/2003PA000912.
Ndung’u, K., M. A. Thomas, and A. R. Flegal (2001), Silver in the western
equatorial and South Atlantic Ocean, Deep Sea Res., Part II, 48, 2933 –
2945, doi:10.1016/S0967-0645(01)00025-X.
Ploug, H., M. Kühl, B. Cleven-Buchholz, and B. J. Jørgensen (1997),
Anoxic aggregates: An ephermeral phenomenon in the pelagic environment?, Aquat. Microb. Ecol., 13, 285 – 294, doi:10.3354/ame013285.
Ranville, M. A., and A. R. Flegal (2005), Silver in the North Pacific Ocean,
Geochem. Geophys. Geosyst., 6, Q03M01, doi:10.1029/2004GC000770.
Reinfelder, J. R., and N. S. Fisher (1991), The assimilation of elements
ingested by marine copepods, Science, 251, 794 – 796, doi:10.1126/
science.251.4995.794.
Rivera-Duarte, I., A. R. Flegal, S. A. Sanudo-Wilhelmy, and A. J. Veron
(1999), Silver in the far North Atlantic Ocean, Deep Sea Res., Part II, 46,
979 – 990, doi:10.1016/S0967-0645(99)00012-0.
Rosenthal, Y., P. Lam, E. A. Boyle, and J. Thomson (1995), Authigenic
cadmium enrichments in suboxic sediments: Precipitation and postdepositional mobility, Earth Planet. Sci. Lett., 132, 99 – 111, doi:10.1016/
0012-821X(95)00056-I.
Sanudo-Wilhelmy, S. A., and A. R. Flegal (1992), Anthropogenic silver in
the southern California Bight: A new tracer of sewage in coastal waters,
Environ. Sci. Technol., 26, 2147 – 2151, doi:10.1021/es00035a012.
Savenko, V. S., and B. R. Tagirov (1996), The physico-chemical state of
silver in seawater (inorganic forms), Oceanology, Engl. Transl., 36, 212 –
215.
Schenau, S. J., M. A. Prins, G. J. De Lange, and C. Monnin (2001), Barium
accumulation in the Arabian Sea: Controls on barite preservation in marine sediments, Geochim. Cosmochim. Acta, 65, 1545 – 1556, doi:10.1016/
S0016-7037(01)00547-6.
Schubert, C. J., T. G. Ferdelman, and B. Strotmann (2000), Organic matter
composition and sulfate reduction rates in sediments off Chile, Org.
Geochem., 31, 351 – 361, doi:10.1016/S0146-6380(00)00005-X.
Shanks, A. L., and M. L. Reeder (1993), Reducing microzones and sulfide
production in marine snow, Mar. Ecol. Prog. Ser., 96, 43 – 47, doi:10.3354/
meps096043.
Smith, I. C., and B. L. Carson (1977), Trace Metals in the Environment,
Ann Arbor Sci., New York.
Taylor, S. R., and S. M. McLennan (1995), The geochemical evolution of
the continental crust, Rev. Geophys., 33, 241 – 265, doi:10.1029/
95RG00262.
Tribovillard, N., T. J. Algeo, T. Lyons, and A. Riboulleau (2006), Trace
metals as paleoredox and paleoproductivity proxies: An update, Chem.
Geol., 232, 12 – 32.
Turekian, K. K., and K. H. Wedepohl (1961), Distribution of the elements
in some major units of the Earth’s crust, Geol. Soc. Am. Bull., 72, 175 –
192, doi:10.1130/0016-7606(1961)72[175:DOTEIS]2.0.CO;2.
van Beek, P., R. Francois, M. Conte, J.-L. Reyss, M. Souhaut, and M. Charette
(2007), 228Ra/226Ra and 226Ra/Ba ratios to track barite formation and
transport in the water column, Geochim. Cosmochim. Acta, 71, 71 – 86,
doi:10.1016/j.gca.2006.07.041.
16 of 17
GB4010
MCKAY AND PEDERSEN: ACCUMULATION OF Ag IN MARINE SEDIMENTS
von Breymann, M. T., K.-C. Emeis, and E. Suess (1992), Water depth and
diagenetic constraints on the use of barium as a palaeoproductivity
indicator, in Upwelling Systems: Evolution Since the Early Miocene,
edited by C. P. Summerhayes et al., Geol. Soc. Spec. Publ. 64,
273 – 284.
Wen, L.-S., P. H. Santschi, G. A. Gill, C. L. Paternostro, and R. D. Lehman
(1997), Colloidal and particulate silver in river and estuarine waters of
Texas, Environ. Sci. Technol., 31, 723 – 731, doi:10.1021/es9603057.
Zhang, Y., H. Amakawa, and Y. Nozaki (2001), Oceanic profiles of
dissolved silver: Precise measurements in the basins of western North
Pacific, Sea of Okhotsk, and the Japan Sea, Mar. Chem., 75, 151 – 163,
doi:10.1016/S0304-4203(01)00035-4.
Zhang, Y., H. Obata, and Y. Nozaki (2004), Silver in the Pacific Ocean and
the Bering Sea, Geochem. J., 38, 623 – 633.
GB4010
Zheng, Y., A. van Geen, and R. F. Anderson (2000), Intensification of the
northeast Pacific oxygen minimum zone during the Bolling-Allerod warm
period, Paleoceanography, 15, 528 – 536, doi:10.1029/1999PA000473.
Zheng, Y., R. F. Anderson, A. van Geen, and M. Q. Fleisher (2002), Preservation of particulate non-lithogenic uranium in marine sediments,
Geochim. Cosmochim. Acta, 66, 3085 – 3092, doi:10.1016/S00167037(01)00632-9.
J. L. McKay, College of Oceanic and Atmospheric Sciences, Oregon
State University, 104 Ocean Administration Building, Corvallis, OR
97331-5503, USA. ([email protected])
T. F. Pedersen, School of Earth and Ocean Sciences, University of
Victoria, P.O. Box 3055 STN CSC, Victoria, BC V8W 3P6, Canada.
17 of 17